1. Field of the Invention
This invention relates to solid electrolytic capacitors using conductive polymers as a solid electrolyte and a number of manufacturing the same, and more particularly to solid electrolytic capacitors ESRs (Equivalent Series Resistances) which are lower in high frequency range than those of conventional solid electrolytic capacitors.
This application is based on Patent Application No. Hei 11-218188 filed in Japan, the contents of which are incorporated herein by reference.
2. Background Art
With the rapid progress in the field of personal computers and other high-tech machines in recent years, the operation frequencies of CPUs and other devices used in them have reached the order of a few hundreds MHz, and therefore in CPUs and other devices, large capacity capacitors have begun to be used as rapidly operable power supplies, which operate at a speed which cannot be achieved with power circuits. Since large capacity capacitors cannot provide power instantly and errors may occur in operation of the CPU and other devices when the ESRs (Equivalent Series Resistances) of the large capacity capacitors are large, the ESRs must be as low as possible.
Solid electrolytic capacitors in the prior art include, for example, those manufactured through the following manufacturing process.
First, a fine powder of a metal such as tantalum, wherein an oxidation film is formed on the surface of the metal particles (a valve action metal), is shaped into a pillar, such as a circular pillar or a rectangular pillar, with an anode lead being provided at one end, the shaped pillar is sintered, and thus a porous anode body with countless small pores in the pillar is obtained. Next, a film of metal oxide as a dielectric layer is formed on the surfaces inside the pores of the anode body and the external surface of the anode body. Techniques such as anode oxidation are used to form the dielectric oxide layer.
Next, a layer of a solid electrolyte such as manganese dioxide is formed on the dielectric oxide layer and then a cathode layer is formed on the solid electrolyte layer. The cathode layer provides a low resistance between the solid electrolyte layer and the external cathode terminal which is made later. The cathode layer is formed, for example, by layering a silver paste layer and a solder layer. Next, this manufactured structure is provided with a cathode terminal for external electric connection, and the structure is encapsulated and sealed by a layer such as a resin layer for molding, and the structure is provided with terminals, and whereby a solid electrolytic capacitor is obtained.
A solid electrolytic capacitor having a porous sintered body as an anode has a large capacity even if its volume is small. However, such a solid electrolytic capacitor has the disadvantage that its ESR is large. The is because that the electric resistances of the solid electrolyte layers are large, because the solid electrolyte layers formed in the small pores are long and thin and MnO2 and other materials used in the solid electrolyte layers are semiconductors.
For this reason, solid electrolytic capacitors in which conductive polymers, which have a lower electric resistance by a factor of a thousand compared with those of MnO2 and other materials, are used, have been developed.
Methods to form a solid electrolyte layer made of such conductive polymers include chemical oxidation polymerization in which monomers for forming conductive polymers are chemically polymerized with chemicals such as oxidants and electrolytic polymerization in which monomers for forming conductive polymers are electrochemically polymerized.
The electric resistances of the conductive polymers produced by the chemical oxidation polymerization are higher (by a few times to a hundred times) than those of conductive polymers produced by the electrolytic polymerization if common monomers for forming conductive polymers are used. For that reason, it is preferable to use the conductive polymers produced through the electrolytic polymerization in order to reduce the ESR in the solid electrolytic capacitors.
However, the electrolytic polymerization has the disadvantage that it is difficult to form conductive polymers produced by the electrolytic polymerization on the dielectric oxide layer because electrolytic polymerization is a kind of electrochemical reaction and cannot be carried out on insulators such as dielectric oxide layers in which electric current cannot flow. For this reason, it is necessary that one conductive polymer (chemically polymerized layer 24) are formed in advance on dielectric oxide layers 22 on a porous anode body 21 by chemical oxidation polymerization, as shown in FIG. 8, and then the other conductive polymers (electrolytic polymerized layer 25) are formed on the chemically polymerized layer 24 by electrolytic polymerization to form a two-layer structure with chemically polymerized layer 24. The electrolytic polymerized layer 25 is difficult to form on the surfaces inside the pores 23 of the anode body 21 and is formed mainly on the external surface of the anode body 21. Such a solid electrolytic capacitor using a solid electrolyte layer made of a two-layer structure of conductive polymers is disclosed, for example, in Japanese Examined Patent Application No. Hei 4-74853.
Although the electric resistance of such a two-layered structure is larger than that of a structure made of only electrolytic polymerized layers 25 since alternating currents must pass through the chemically polymerized layer 24, the electric resistances of the chemically polymerized layer 24 was not considered to present a significant problem because the chemically polymerized layer 24 is indispensable as an underlayer for the electrolytic polymerized layer 25 and the chemically polymerized layer 24 has a much lower resistance than the prior art layers such MnO2.
However, solid electrolytic capacitors using a solid electrolyte layer made of a two-layered structure of conductive polymers, when used at high frequencies (about 100 kHz), have the problem that their ESRs are larger than those of the prior art. The large ESP results from the following mechanism. Electric current passes through the porous anode body 21, the chemical polymerized layer 24, the electrolytic polymerized layer 25, and the cathode 26 in turn, as shown in FIGS. 9 and 10, and particularly at high frequencies, electric current passes only through the parts of those constituent elements which are located near the outer surface of the porous anode body 21, and thereby the resistance of the parts of the chemical polymerized layer 24, which are located near the outer surface of the porous anode body 21, have a large effect on the total resistance. In view of the need that has risen in recent years for solid electrolytic capacitors with low ESRs, it has become important to reduce the ESR values even if only by a few mxcexa9 to 10 mxcexa9. For this reason, it is desired to obtain solid electrolytic layers with the lowest resistance possible.
The purpose of the present invention is to provide solid electrolytic capacitors with low ESRs in the high frequency range and methods for manufacturing the same.
The solid electrolytic capacitors of the present invention comprise a porous anode body made of a valve action metal and on the surface of which body a dielectric oxide layer is formed; and a chemically polymerized layer made of conductive polymers which are formed by chemical oxide polymerization on the dielectric oxide layer located on the surfaces inside the pores of the porous anode body; and an electrolytic polymerized layer made of conductive polymers which are formed by electrolytic polymerization on the dielectric oxide layer located on the external surface of the porous anode body; wherein the dielectric oxide layer and the electrolytic polymerized layer are in contact with each other and the chemically polymerized layer and the electrolytic polymerized layer are electrically connected with each other near the contacting parts.
The method for manufacturing solid electrolytic capacitors of the present invention comprises the steps of: forming a dielectric oxide layer on a surface of a porous anode body which is made of a valve action metal; and forming a chemically polymerized layer made of conductive polymers by chemical oxide polymerization by means of repeating more than one cycle of the following two substeps, one substep being immersing the porous anode body with the dielectric oxide layer in a solution which includes 10% to 60% oxidant per weight of the solution, and then drying the porous anode body with the dielectric oxide layer, and the other substep being immersing the dried porous anode body with the dielectric oxide layer in a solution including monomers for forming conductive polymers and then cleaning the immersed porous anode body with the dielectric oxide layer; and immersing the porous anode body with the dielectric oxide layer and the chemically polymerized layer in a solution including dopants and monomers for forming conductive polymers and forming an electrolytic polymerized layer made of conductive polymers by electrolytic polymerization.
In the method for manufacturing solid electrolytic capacitors of the present invention, it is preferable that the solvent used for the solution including an oxidant is water or methanol.
Additionally, in the method for manufacturing solid electrolytic capacitors of the present invention, it is preferable that the porous anode body with the dielectric oxide layer is cleaned with water or methanol between the two substeps, one substep being immersing the porous anode body with the dielectric oxide layer in a solution which includes an oxidant and then drying the porous anode body with the dielectric oxide layer, and the other substep being immersing the dried porous anode body with the dielectric oxide layer in a solution including monomers for forming conductive polymers and then rinsing the porous anode body with the dielectric oxide layer.